patch 9.1.1136: Match highlighting marks a buffer region as changed

patch 9.1.1136: Match highlighting marks a buffer region as changed

Problem:  Match highlighting marks a buffer region to be redrawn as if
          its buffer text was changed, unnecessarily invoking syntax code.
Solution: Set the `w_redraw_top/bot` variables instead of the b_mod_* ones
          (Luuk van Baal)

closes: #16697

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>